My only problem with btrfs is the lack of a fsck tool that actually corrects errors, otherwise btrfs was working great until something went wrong and I couldn't correct errors in it. However, does space_cache deal with the huge about of "wasted space" that a btrfs system has?
What? ZFS does not have any "fsck" tool, it need not by design. I think it is the same with BTRFS, it need no "fsck"?